1. Field of the Invention
The present invention relates to a laser transmitter capable of being configured to transmit one of a plurality of wavelengths. Specifically, the laser transmitter may be reconfigured using the resonance passbands of a microresonator.
2. Description of Related Art
More and more applications are using photonic links to transmit signals. For example, in antenna remoting applications, a RF signal can be modulated onto an optical carrier and subsequently sent over an optical link to a remote site for possibly additional processing and for remote RF wave transmission from an antenna or an antenna farm. Wavelength Division Multiplexing (WDM) technologies have much to offer in this application. First, many signals can be multiplexed into one strand of optical fiber via WDM, thus alleviating the need to deploy a large number of fibers for the remoting of an antenna farm. Second, the WDM channel multiplexers hold a tremendous advantage in insertion loss over conventional optical combiners as the number of multiplexed channels increases. For example, an 8:1 WDM multiplexer will demonstrate an optical insertion loss of only 5 dB. In comparison, a conventional optical power-combiner has an optical insertion loss of about 10 dB. For 16:1 and 32:1 multiplexing, the combined losses of WDM multiplexers stay constant at approximately 7 dB.
Transmission of signals via photonic links may also be used in a number of other applications. For example, many systems contain a mixture of analog and digital data that originates from a variety of sensors. Typically, the outputs of these sensors are linked together with affiliated processors and displays via a versatile and reconfigurable network. As discussed above, WDM optical networks offer bandwidth enhancements with a concurrent reduction in the physical number of cables and connectors. These extra cables and connector interfaces represent potential on-board service-points. Minimization of these on-board service-points increases reliability. Reconfigurable WDM networks and components provide a platform with the capabilities to perform changing missions, address changing environments, and reduce the variety of components stocked in the maintenance inventory. Another desire, for some systems, is to reduce the overall power consumption.
The International Telecommunications Union (ITU) has specified a grid of wavelengths (near λ˜1550 nm) as the standard carrier frequencies for Wavelength Division Multiplexing (WDM) networks. Depending upon the desired number of transmission channels, these wavelengths (a.k.a. ITU-channels) are designed to be 50 GHz (=0.4 nm) or 100 GHz (=0.8 nm) apart. By standardizing the carrier frequencies, the ITU hopes to ensure that a WDM network's key passive components, such as wavelength multiplexers/demultiplexers, routers/switches, etc., are spectrally compatible with one another, even though they might be developed by different manufactures. Hence, the emission wavelengths of WDM transmitters are designed to align precisely with the ITU-grid.
Previously, distributed feedback (DFB) or distributed Bragg reflector (DBR) laser structures were commonly employed to provide WDM transmitters with emission wavelengths that align precisely with the ITU-grid. DFBs or DBRs provide single mode laser diodes that emit a wavelength near an ITU-channel. Thermal tuning (at a rate of 0.1 nm/° C.) is then used to move the diode's lasing wavelength into precise alignment with the desired ITU-channel. Finally, an external passive element, typically a Fabry-Perot etalon, is used in a feed back loop as a “wave-locker” to maintain the precise alignment of the wavelength over the long term.
There are several disadvantages to the prior art approach. First, the tuning speed via thermal heating/cooling is slow, typically in the millisecond range. Second, the number of distinct DFB/DBR laser “models” needed multiplies rapidly as the number of wavelength channels increases. In addition, each DFB/DBR laser “model” requires a slightly different fabrication procedure. Finally, the wavelength control feedback loop with the “wave-locker” is physically cumbersome, increasing the size of the transmitter package substantially.
To reduce the inventory count of discrete diode elements in a WDM network, various types of wavelength tunable transmitters have been developed in recent years. These include external cavity lasers, as shown in FIG. 1a, whose emission wavelengths are controlled mechanically via the physical alignment of an external grating. The physical alignment of the external grating is made with respect to the optical feedback path in the laser cavity. In other designs, as shown in FIG. 1b, the wavelength, λB (mλB=2Λneff, where m is an integer) of the distributed Bragg reflector (period=Λ) in the diode laser is electrically tuned, via current-induced index (neff) changes, to vary the lasing wavelength. However, these tunable DBR lasers require the use of a phase control-section, in addition to the gain and Bragg sections, to accomplish the quasi-continuous wavelength tuning. Furthermore, the implementation of complex multi-variable algorithms is often necessary to guide the bias-currents of all three diode sections, so that the alignment of wavelength, λB to the ITU-grid can be accomplished and maintained. For more information see Sarlet et. al, “Wavelength and Mode Stabilization of Widely Tunable SG-DBR and SSG-DBR Lasers”, IEEE Photon. Technol. Lett., Vol. 11, No. 11, 1999, pp. 1351-1353.
Another prior art solution can be found in J. Berger, et. al, “Widely tunable external cavity diode laser based on a MEMS electrostatic rotary actuator”, Paper TuJ2-1, OFC 2001, Anaheim, Calif. This solution utilizes a MEMS-tuned external cavity diode laser as shown in FIGS. 2a and 2b. To achieve compactness, the optical feedback path of this external cavity laser is folded, making the assembly and alignment of the laser chip, lens, diffraction grating, and MEMS-controlled mirror extremely critical for optimal performance. In addition, a relatively large voltage, approximately 140 volts, on the MEMS actuators is needed to rotate the mirrors by ±1.4°. As with other external cavity lasers that are tuned mechanically, the response speed for “hopping” from one ITU-channel to another is slow. Specifically, it takes 15 msec for the MEMS actuators to execute a coarse tuning towards one of the ITU-channels. Finally, “wave-lockers”, used in conjunction with software algorithms, are needed to fine-tune these coarse set points to a precise alignment with the ITU-grid.